Thursday, 18 February 2016

A question I am often asked is why steam appears to be so much more effective than hot water in causing burns. Thermal burns are caused by a momentary contact with a hot agent - water, steam, hot iron. Our body reacts very quickly to move away - the contact time would be of the order of our reaction time; let us say the reaction time is tenth of a second or 0.1 sec. During this short time, heat energy is transferred to the local spot on our outer skin (epidermis) and raises its temperature to cause the burn. Slides at the end of the blog provide information about the structure of human skin. The reason steam causes severe burns is because it carries much more energy than water - we shall return to this after some preliminaries.Epidermis starts to get damaged at temperatures above 44 C. If the temperature of water is higher, then more heat energy will be transferred to the tissue and the damage will happen more quickly and severely. One talks of damage to the skin in terms of the degree of burn - first degree burn is mildest while the fourth degree burn is really severe life threatening. Slides at the end of this blog tells you about classification of burns.For third degree burns to happen, the time of contact is as follows:

1 second at 69 C water temperature2 seconds at 65 C

5 seconds at 60 C

15 seconds at 56 C

Notice that the time for burn is not linear and reduces rapidly as the temperature of hot water increases.

For boiling water at 100 C it takes much less than 0.01 sec for burn to happen - third degree burn is very serious - first and second degree burn happen for even shorter contact times.

Once you splash boiling water droplets on your skin or touch a hot iron, a burn shall happen. The heat energy is rapidly conducted away to surrounding tissue and this limits the size of the burn. If you splash lot of boiling water then area of contact is greater and heat conduction from the central part is poor and the resulting burn is more serious in the centre. Hot oil causes even more serious burns because oil tends to be hotter (greater than 100 C) and is sticky - does not fly off the skin as rapidly as water does.

Before looking at scalding by steam, let us consider the threshold energy that can cause burns. I am able to find data for arc lamp induced second degree burns. Since skin burns mainly depend on the temperature increase of the skin, the data can be used to get some idea about the amount of heat required to cause a second degree burn by water as well. The IEEE P1584 and NFPA 70E standards state that a second degree burn is possible by an exposure of unprotected skin to an electric arc flash above an incident energy level of 5 J per square cm.The figure shows the time of burn for different energy flux (amount of energy delivered to one square cm of the skin per second). The higher the energy flux, the shorter is the time for second degree burn.

Now, we are ready to talk about steam and hot water burns.The idea here is that both hot water and steam deposit energy on the skin but the rate of this energy deposition is greater for steam than it is for water because steam carries much more energy than hot water. To explain this, we need to do some interesting physics.Think what happens when you heat water in a container: Water temperature increases steadily until it reaches 100 C. The temperature stays at 100 C but some water is converted to steam - the conversion continues until all of the water has changed into steam - temperature stays at 100 C. Where is this heat energy going - it is being used up in breaking the bonds between different water molecules and making them free. This energy is called the latent heat of vaporisation of water. Latent - because the energy is hidden and has not resulted in a temperature change. This is shown in the slide

The interesting thing is that to raise the temperature of 1g of water from 0 to 100 C requires 418 J of energy. But to convert 1g of water at 100 C to steam requires 2260 J of energy -- 5 times more energy.

If water or steam touches the skin then its temperature will drop very quickly to about 40 to 50 C (Skin temperature is about 36 C) and 1g of water at 100 C will give up 209 J of energy to the tissue. Steam will give up the latent heat + 209 J or 2469 J of energy to the tissue - this is almost 10 times more energy. But we must remember that steam is much lighter than water and we shall receive only a small amount of steam on the skin.

I think the actual situation is as follows:Boiling water is a mixture of water at 100 C mixed with lot of steam. Burns due to boiling water are exacerbated by the presence of steam. Water at a lower temperature - say 70 or 80 C will have no steam mixed with it and a drop of lower temperature water will not cause as serious burn as a drop of boiling water.